Various coagulating agents which can be used for treating effluents, residual waters, or waste waters are known. Examples include ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, and ferric chlorosulfate. Mention may also be made of basic aluminum chlorosulfates such as described in French Patent Application Nos. 2 036 685 and 2 418 297.
Mention can also be made of the basic aluminum chlorosulfates described in French Patent Publication Numbers 2 584 699, 2 600 321, and 2 600 322, which are described as providing a basic aluminum chlorosulfate which can provide low levels residual aluminum in treated waters, capable of forming muds having high coefficient cohesion and which are both colorless and have a low level of cloudiness.
The basic aluminum chlorosulfate as described in the previously referenced French Patent Publications comprises a substance of the formula: EQU Al.sub.n OH.sub.m (SO.sub.4).sub.k Cl.sub.3n-m-2k I
in which the basicity or the ratio m divided by 3n times 100 is between 40% and 65%, and which has an aluminum equivalent/chloride equivalent ratio of between 2.8 and 5, an Apparent Molecular (AM) mass measured by conventional light diffusion in aqueous solution of 8.3% Al.sub.2 O.sub.3 by weight within the range of from about 7,000-35,000 and preferably 22,000 and 32,000 daltons, said basic aluminum chlorosulfate being characterized as having apparent hydrodynamic diameters Phi.sub.Z and Phi.sub.W measured by quasi-elastic diffusion of light, of the following values expressed in manometers:
Phi.sub.Z (nm)=35-250 (350-2500 .ANG.) and preferably 80-100 nm PA0 Phi.sub.W (nm)=20-120 (200-1200 .ANG.) and preferably 150-220 nm
As disclosed in the previously referenced French Patent Publications, the basic aluminum chlorosulfate of formula I is prepared by a process comprising the steps of reacting a slurry of calcium chloride with calcium carbonate to form a chlorocarbonated slurry, bringing the chlorocarbonate slurry into contact with aluminum sulfate, wherein the slurry is incrementally added over a period ranging from about 45 minutes to four hours and separating the resulting solid by-product from the reaction medium. This separation makes it possible to obtain a calcium sulfate filter cake and a filtrate containing the basic aluminum chlorosulfate. The amounts of the chlorocarbonated slurry and the aluminum sulfate which are added are such that, for the basic aluminum chlorosulfate produced, there is a ratio m/3n times 10% of between about 40% and 65% and an aluminum equivalent/chloride equivalent ratio between 2.8 and 5.
The patent states the importance of three parameters: basicity of between about 40% and 65% and more particularly between about 45% and 56%; Al/Cl equivalent ratio of between about 2.8 and 5, preferably 3.2 to 5 and more preferably 3.2 to 4; and the degree of desulfation (said degree representing the ratio of the amount of sulfate removed to that present in the initial aluminum sulfate) of between about 70% and 90%, preferably 70%-85% and more preferably between about 75% and 82%. These three parameters are stated to be important in preparing a product having a high level of stability represented by storage time in the absence of precipitation. The patent teaches that the product disclosed therein has a level of stability of several months at ambient temperature.
Finally, measurements by NMR of the monomer aluminum showed that there is from 15% to 25% of monomer aluminum in the basic aluminum chlorosulfate, the rest of the aluminum being in polymer form.
The products may also be characterized in an equivalent and equally applicable manner by means of their mean molecular mass by weight M.sub.w and their mean real hydrodynamic diameters Phi.sub.Zt and Phi.sub.Wt.
Measurement in respect of those values is effected in conventional fashion by extrapolation at zero concentration of the curves representing respectively the inverse of the apparent molecular mass AM and the inverse of the apparent hydrodynamic diameters in dependence on the concentration of the polymerized fraction of basic aluminum chlorosulfate in question. The correspondence between the real mean value and the apparent value of the molecular mass is in fact given by the Debye relationship as follows: EQU 1/M.sub.w =1/AM+Bc
in which c represents the level of concentration of the solution of basic aluminum chlorosulfate and B represents a constant (coefficient of the virial).
In attempting to prepare the basic aluminum chlorosulfate in accordance with the previously referenced French Patent Publications, it was noted that extremely large amounts of by-product calcium sulfate are formed in the process, equipment expenditure is high, and the product stability is poor as concerns long term storage. If the user of the process disclosed in the French Patent Publications has no outlet for the calcium sulfate mud produced as a by-product, the cost of eliminating the by-product seriously affects the economics of the process. Similarly, equipment required for the many stages of the referenced process makes the process less desirable from a commercial standpoint. The problem of storage stability becomes of great importance if the product remains in storage more than a few months prior to use, as the product precipitates from its normally liquid state to form a non-usable solid. Further, raw materials of pure form having no metallic impurities must be used as the impurities affect the polymerization reaction.
It has now been found that basic aluminum chlorosulfates can be prepared by an improved process which reduces the quantity of by-product calcium sulfate mud, utilizes equipment and raw materials of lower cost, and which provides a more stable product.